a) Field of the Invention
The present invention relates to a technique of distance detecting, and more particularly to a technique of distance detecting according to a phase difference detecting type for detecting the distance to an object, particularly suitable for use with image pick-up apparatuses such as cameras.
b) Description of the Related Art
A correlation type distance detecting method is known which detects a distance to an object by using the triangulation method, which is based upon the relative position of two images of the same object focused on a pair of line sensors. This method can measure a distance by a single range finding operation, and so it is suitable for use with an autofocusing camera.
FIGS. 6A and 6B show an example of a conventional TTL (through the lens), phase difference detecting type range finding device. FIG. 6A shows an example of the structure of the device, and FIG. 6B shows an example of the signal processor of the device. This device will be described as applied to a focus detecting device for a camera by way of example.
Light from an object to be picked up is converged by an imaging lens 51, passed through a film equivalent plane 52 to a separator lens 54 via a condenser lens 53. The separator lens 54 splits the incident light into two light beams and projects them onto a standard line sensor 55 and reference line sensor 56. An image of the object on the optical axis of the imaging lens 51 is transformed into two images by the separator lens 54 and focused on the line sensors 55 and 56.
The line sensor 55 has p photosensor elements and is called a standard line sensor because it is used as a standard. The line sensor 56 has q photosensor elements greater than p, and is called a reference line sensor. Image signals for p photosensor elements of the standard line sensor 55 are read at a fixed read phase, whereas image signals for p photosensor elements of the reference line sensor 56 are read at a sequentially changing read phase. The latter phase can be defined as a relative pixel position to that of the standard line sensor 55. The two image signals are compared to obtain a phase difference therebetween.
The image signals picked up from the standard and reference line sensors 55 and 56 are supplied to a signal processor 57. While sequentially changing the read phase for the image signal picked up from the reference line sensor 56, the signal processor 57 calculates a correlation factor to be described later to detect an extreme value of correlation factors, and hence a distance to an object.
Another range finding method has also been proposed, in which an external light is passed not through an imaging lens 51 but through a pair of lenses having the same characteristic and disposed respectively in front of the standard and reference line sensors, for measurement of a distance to an object.